This invention relates to a device for limiting the input torque of an infinitely variable transmission used in vehicles.
Tokkai-Hei 10-267117 published by the Japanese Patent Office in 1998 discloses an infinitely variable transmission (IVT) which enables control of a speed ratio to infinity by combining a continuously variable transmission (CVT) and a planetary gear set, in order to enlarge the speed change region of the continuously variable transmission.
In this infinitely variable transmission, the speed ratio (referred to hereafter as IVT ration ii) of the infinitely variable transmission is continuously controlled according to the difference of the speed ratios of the CVT and a fixed speed ratio transmission, from a negative value to a positive value including an infinite (geared neutral point GNP). At the geared neutral point GNP the output rotation of the IVT is zero, and the output rotation of the IVT changes direction.
Further, in this IVT, the CVT is controlled to reduce change-over shocks, such as due to the operation of a selection lever, when acceleration starts in the vicinity of the geared neutral point.
In this control, the CVT is controlled so that the output shaft torque of the IVT is suppressed in a predetermined time from when there is a change-over from a non-running range to a running range.
However, in the aforesaid IVT, as the speed ratio is infinitely large at the geared neutral point, the torque amplification factor from the input shaft to the output shaft is a very large value. The input and output shafts of the CVT are connected via a planetary gear set and fixed speed ratio transmission mechanism, so the IVT output shaft torque which is largely amplified in the vicinity of the geared neutral point also largely influences the input shaft.
Therefore, when the engine connected with the CVT generates a large engine torque in the vicinity of the geared neutral point, a very large torque will act on the parts of the CVT. In the aforesaid prior art IVT, when the output torque of the IVT becomes a large value, the CVT tends to be controlled to avoid this.
However, as the engine output torque response characteristics are generally steeper than the speed ratio response characteristics of the CVT, the durability of the CVT may decrease and the desired speed ratio may no longer be obtained due to the deformation of the parts of the CVT when the engine torque input to the CVT is too large.
It is therefore an object of this invention to prevent excessive torque from being input into an infinitely variable transmission when the speed ratio of the infinitely variable transmission is close to the geared neutral point.
In order to achieve above objects, this invention provides an input torque limiting device for an infinitely variable transmission connected to an engine of a vehicle, into which the torque from the engine is input and which can vary a speed ratio to infinity, the limiting device comprising a sensor for detecting an output/input speed ratio of the infinitely variable transmission which is an inverse of the speed ratio and a microprocessor.
The microprocessor programmed to compute an input torque limit according to the output/input speed ratio of the infinitely variable transmission as the maximum torque which can be input without adversely affecting the durability and performance of the infinitely variable transmission, and control the engine torque not to exceed the input torque limit.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.